Among the 7 million stroke survivors living in the U.S., fewer than 10% have adequate walking speed and endurance to complete normal daily activities like grocery shopping, which leads to a loss of life roles. This limitation in walking capacity is caused by both neurologic gait impairments from stroke and also by aerobic deconditioning due to inactivity. Current stroke rehabilitation guidelines recommend moderate-intensity aerobic training (MAT) to address both of these issues. However, recent evidence suggests that a clinically feasible MAT duration (4 weeks) may have only negligible effects among chronic stroke survivors (>6 months post stroke). There is growing belief that a more vigorous training intensity may be a ?critical ingredient? for eliciting greater improvements in both gait and aerobic fitness, with less training time. Yet, the optimal training intensity has been difficult to determine among stroke survivors because neurologic impairments make it challenging to reach vigorous intensity. This study will use a novel training protocol to overcome this limitation. Based on the well-tested exercise science and cardiac rehabilitation strategy of high-intensity interval training (HIT), this protocol uses bursts of maximum speed walking alternating with recovery periods, to sustain higher aerobic intensities than physiologically possible with continuous exercise and with less perceived exertion. Preliminary data demonstrate that this innovative locomotor HIT protocol can elicit remarkable increases in walking capacity, gait speed and aerobic fitness in just 4 weeks among chronic stroke survivors. However, no previous studies have compared HIT to the current best-practice model post-stroke (MAT). Further, it is possible that the longer 12 week HIT durations used in some heart disease studies might yield even better outcomes, approaching normal walking capacity, but no stroke studies have tested HIT for more than 4 weeks. Therefore, the objective of this proposal is to determine the optimal training intensity and the minimum training duration needed to maximize immediate improvements in walking capacity in chronic stroke. Persons >6 months post- stroke at 3 research sites across the U.S. will be randomized to either 12 weeks of HIT or 12 weeks of MAT. Walking function, aerobic fitness, daily activity and quality of life will be assessed at baseline and after 4, 8 and 12 weeks of training. At study completion, we expect to have gained foundational information to guide dosing of locomotor training intensity and duration in future studies and clinical practice, thus making a positive impact on the disability burden of stroke.!